Surface treatment of metal to prevent localized corrosive attack



Patented Jan. 25, 1938 UNITED STATES PATENT OFFICE SURFACE TREATMENT OF METAL T Pm:- VENT LOOALIZED co moslve ATTACK NoDr'awing. Application April 29, 1935,

Serial No. 18,913

16 Claims. (c1. 148 6.5)

This invention relates to the surface treatment of metals in order to render the same immune to localized corrosion when subjected to the attack of corrosive media, among which are chloride and sulphate solutions.

The use in the chemical and allied industries of semi-noble corrosion resisting ferrous alloys containing relatively high percentages of chromium and chromium and nickel such as alloys of M the 18-8 type, whether or not small amounts of other alloying elements are present, has assumed large proportions during the last few years.

these alloys are in the main suitable to the uses to which they have been put, it has been found that all of them, so far as we are aware, are subject to localized corrosion when subjected to the attack of corrosive media among which are chlorideand sulphate solutions.

These alloys, when made up into the form of 00 sheets, plates or other articles, are subjected to acid pickling treatments in order to remove the oxide or scaleformed during annealing operations. Often, the sheets, plates or other articles,

beside being pickled, are ground and polished.

ing scale.

Even after the most approved pickling procedure and even when such procedure is followed by grinding and polishing, or when sand blasting is used for scale removal, the sheets, plates or other articles made from these alloys are still subject to localized corrosion. When examined with the eye, their surfaces may appear perfectly clean, and free from scale or other non-metallic intrusions, but when viewed with a-glass, they show that foreign particles have not been entirely removed. The areas in which these foreign particles are present are areas that are susceptible to localized attack when subjected to corrosive media, among which are solutions containing chlorides, sulphates and allied salts.

The effect of the condition of the surface of these alloys has been generally recognized and many attempts have been made to so prepare or 5 treat the same as to immunize them against such localized attack. I

It is accordingly one of the objects of our present invention so to treat corrosion resistant steels, e. g., of the high chromium and high chromium nickel types, that their existing tendency to pit or corrode locally in the presence of certain media will be eliminated.

It is another object of our invention to remove surface intrusions from these steels due to prior treatments while at the same time causing the Some are sand blasted for the purpose of remov- I natural protective film thereon to become both continuous and of improved characteristics.

A further object of our invention resides in speeding up the natural tendency of these steels to generate protective films thereon while removing or eliminating the causes for the generation of a Weak discontinuous film.

More specifically, an object of our invention resides in treating the surface of corrosion resistant steels of the chromium and chromium nickel types with a bath of chromic and hydrofluoric acids in aqueous solution under predetermined conditions.

Other objects and advantages will be understood by those skilled in this art.

It is now a generally accepted fact that the corrosion resistance of the high chromium and high chromium nickel alloys embraced in the group which we term semi-noble is due to the presence of a protective film which automatically forms on the surface of such alloys. This film is believed to be not over a few atoms in depthand its automatic formation is an inherent propertyof such alloys which other alloys not containing I high chromium and high chromium and nickel do not possess.

Because of this protective film, the surface layers of sheets, plates or other articles made'from these alloys are different from the metal beneath the surface and it is believed that it is the peculiar condition of these surface layers that renders these alloys immune to attack under severe 'corrosive conditions. I

This protective film which normally and automatically forms, is not continuous because of the intrusions which remain not only after pickling,

, protected by this natural automatically formed film and these are the areas or portions which corrode when the metal is subjected to the action of corrosive media. This corrosion leads to the formation of pits.

Apparently there exists quite a difference of potential between the surface metal covered by this natural film and the surface metal not covered by the same. There is, therefore, set up a minute electrolytic cell at each surface intrusion or at each portion of the surface where such nat 7 ural film is interrupted or broken. In. such cell the metal which is protected by the film is more electro-negative than the metal which is not protected, so that that portion of the metal not protected by the film becomes attacked. When these cells are set up, the rate at which pitting progresses depends upon the products of the electrol ysis formed. The rate therefore at which this dissolution takes place depends upon the nature of the media to which the surface is subjected- For example, in solutions which do not contain salts of the strong-mineral acids, the rate of attack is considerably less than in solutions which contain such salts as sulphates and chlorides. On the other hand, in solutions containing nitrates, the rate of attack is considerably less than in those containing sulphates and chlorides. It is quite apparent that this should be the case, since when an electrolytic cell is once formed, we have, in the case of the first two, the electrolysis of sulphates and chlorides which invariably yields small quantities of hydrochloric and sulphuric acid. 'These acids are of course formed at the electro-positive pole of the cell and the attack becomes very rapid, possibly with the formation of ferrous chloride, if iron is present (in the case of chloride solutions) and in the case of sulphate solutions either ferric or ferrous sulphate, depending upon the conditions. In the case of nitrates, nitric acid is formed. These corrosion resistantalloys are not readily attacked by nitric acid, so that in the case of nitrate solutions, the pitting efiect is much less.

This phenomenon of localized corrosive attack is manifested in service in many different types of corrosive media and in a number of these media, I

hibitive. Pits, however, are slowly formed and these cut down the life of the alloy in service.

In chloride, sulphate and allied solutions however, the products of electrolysis are such that the metal is very rapidly destroyed by the formation of pits, so that for applications of this kind, unless a chemical is continuously added to neutralize the products of corrosion as formed, these semi-noble corrosion resistant alloys have such a short life that their use for services in which.

they are subjected to the attack of such solutions is unwarranted.

It is reasonable to expect that if a continuous and uninterrupted protective surface film can be formed, these alloys will not be susceptible to localized corrosive attack and an object of this invention is to provide a method for obtaining a continuous protective film such as will render ferrous alloys of the semi-noble type immune to localized corrosive attack.

In carrying out the method of our invention, we prepare a bath containing from 1-20% chromic acid (chromic acid anhydride) 98% by weight, and from 1-10% hydrofluoric acid calculated at 48% by volume, the remainder of the bath being made up of water. The temperature of the bath is preferably raised to from about to about- 180 F. and the articles to be treated are immersed therein for a period of about twenty minutes. The lower the temperature, the longer the time,

required and conversely the higher the tempera ture, the shorter the time required.

. The articles to be treated may be sheets, plates or articles made therefrom or articles such as rods, etc., and it will be understood that it is preferable to have the surfaces of these as free from scale and/or oxide as it is possible to get the same polishing after pickling or after sand blasting.

We have found that when these corrosion resistant ferrous alloys are treated in accordance with this invention, they no longer exhibit any tendency to pit in chloride and sulphate solutions.

We find that all surface intrusions are removed and that an effective, continuous and uninterrupted protective film is formed causing the surface of the treated articles to have a uniform potential.

Apparently our treatment not only removes the intrusions and produces a continuous surface film, but also-changes the nature of the surface film so that it is more strongly protective than the films which are automatically formed by nature. I We have found that a solution containing 4% chromic acid (98% by weight) and 4% hydrofluoric acid (calculated at 48% by volume) at F. will produce the desired effect upon alloys of the 18--8 type in about 20 minutes.

We have found that when articles made from alloys of the 188 type are treated in this manner, no appreciable metal loss takes place, indicating to us that the surface of the article treated, undergoes practically no attack as in a pickling operation. The foreign particles which break down the natural surface film are removed and the protective film is greatly improved in character.

The effectiveness of the method of our invention in preventing localized attack is clearly shown by the following.

We have run tests, using a 10% ferric chloride solution, inasmuch as in this type of attack, this compound is invariably formed. Sixteen gauge sheets made from an 18-8 type of alloy which had been thoroughly pickled and processed in the regular way, that is, in accordance with the procedures set forth in Kiefer Patents Nos. 1,974,570 and 1,974,571 of Sept. 25, 1934, were subjected to a 10% ferric chloride solution at room temperature. Pits started in a very short time as is invariably the case and in 16-20 hours, some of these pits extended completely through the sample.

Other sheets of the same alloy were subjected to a solution containing 4% sodium chloride with only a few tenths of 1% ferric chloride. In this case, the results were the same as in the former case.

Samples of this same material were treated in These remained unattacked in this solution, even after 100 hours, whereas the companion samples taken from the same sheet but untreated, pitted through in a few hours.

,. The effectiveness of our method is further evitrated completely through thecontainer in from 10 to 24 hours, whereas, containers of the same alloy, after being treated in accordance with our invention, did not show any. attack or pitting when exposed'to the same solution for 100 hours.

We have practiced our method on many hundreds of samples of the 18--8 type of alloy obtained from difierent heats of metal which were pickled and processed by different methods and we found that the samples treated showed ,practically no pitting when subjected for long periods to ferric chloride or salt solutions containing ferric chloride, whereas companion samples taken from the same sheets which had not been treated, showed very bad pitting in a relatively short time.

It is therefore apparent that the localized cor-- rosion or pitting of corrosion resistant stainless alloy-steels, e. g., those of the 18-8 type, can be explained on the basis thatthe natural protective film which automatically forms upon the alloy is not only exceedingly thin, but is discontinuous. The discontinuity results primarily from the fact that due to the procedures followed in finishing the steels, a number of intrusions areleft in the surface of the steels. Thus, when the -Our present treatment, as above described,

overcomes these defects and disadvantages so that steels are produced which have surfaces free from intrusions and which have not only continuous natural films, but films which are of improved characteristics, i. e.', as to tenaciousness, strength, thickness, and resistance to chemical or electrolytic action. The bath of chromic and hydrofluoric acids acts both to eliminate or remove the intrusions and to speed up the forma tion of a continuous protective film while at the same time altering or improving the characteristics of the film, especially in the particulars abovereferred to. 'Our treatment does not act to deposit a coating or layer of material on the steels, but on the contrary, by removing the intrusions, makes it possible for the natural protective film to become continuous and simultaneously speeds up the formation of the film while improving its tenaciousness, strength, and stability. We thus remove the intrusions in the presence of a material which already exists in the natural protective film. By this we mean that the natural film contains chromic oxide and the bath likewise contains chromic oxide, but

careful study, observation and test convinces us that there is no actual deposit of chromic oxide from the bath on the surface of the alloy. It is "also apparent that our bath consists only of chromic and hydrofluoric acids in aqueous solution and that our bath contains no ingredients which will either adversely affect the steels themselves or which will in any way counteract or minimize the special-functions of the chromic and hydrofluoric acids.

What we claim as new and desire to secure by Letters Patent is:-

1. A method of treating articles made from a corrosion resistant ferrous alloy of the l8-8 type in order to immunize the same against localized corrosion, which consists in subjecting such articles to the action of a bath composed of 1-20% chromic acid and hydrofluoric acid in.

an amount equivalent to from 1-10% of 48% (by volume) acid, the balance being water.

2. A method of treating articles made from a corrosion resistant ferrous alloy containing about 18% chromium and about 8% nickel in corrosion, which consists in subjecting such articles to the action of a bath composed of -1-20% chromic acid and hydrofluoric acid in an amount equivalent to from 1-10% of 48%v (by volume) acid the chromic acid being so proportioned to order to immunize the same against localized the hydrofluoric acid as to preventapprcciable attack of the alloy, and the balance 'of the bath being water. I

3. A method of treating articles made from a corrosion resistant ferrous alloy in order to immunize thesame' against localized corrosion.

-which consists in subjecting such articles to the action of a bath composed of l20% chromic acid (98% by ,weight), 1-10% hydrofluoric acid (calculated at 48% by volume) and the remainder water.

4. A methodof treating articles made from a corrosion resistant ferrous alloy in order to im-- munize the same against localized corrosion,-

which consists in subjecting such articles to the action of a bath containing about 4% chromic acid (98% by weight) and about 4% hydrofluoric acid (calculated at 48% by volume) at a temperature of from about 120 to about 180 F.

5. The method of obtaining a surface of uniform potential 'on an article made from a corrosion resistant ferrous alloy, which consists in subjecting the article to the action of a bath composed of 1-20% chromic acid and hydro: fluoric acid in an amount equivalent to from 1-10% of 48% (by volume) acid, the balance being water, for a length of time suificient to provide an uninterrupted protective film covering the surface so treated.

6. A method of treating sheets, plates and the like formed from acorrosion resistant ferrous alloy, which consists in removing scale and oxide from the same and thereafter subjecting them to the action of 'a solution composed of '120% chromic acid and hydrofluoric acid in an amount equivalent to from l-l0% of 48% (by volume) acid, the balance being water.

7. A bath for the treatment of articles made from a corrosion resistant ferrous alloy, which consists of ll0% commercial hydrofluoric acid,

characterized by freedom from any constituent having a-harmful influence and having the hydrofluoric and chromic acids so proportioned 1-20% chromic acid and water; the bath being I that articles treated therein will be free from apwhether or not they contain small amounts of other alloying elements, which consists in subjecting such articles to a bath composed of 1-20% chromic acid, l10% commercial hydrofluoric acid and water and being characterized by freedom from any constituent which impairs the effectiveness of such acids in promoting the formation of such protective surface film.

9. A method of treating articles made'from alloys having a relatively high chromium content or a relatively high chromium and relatively high nickel content whether or not such alloys contain small amounts of other alloying elements, which consists in subjecting such articlesto a scale removing procedure, and in providing sucharticles with a surface film of such character as to render the same immune to localized corrosive attack by subjecting the same to the action of a bath consisting of 1-20% chromic acid, hydrofluoric acid in an amount equivalent to from 1-10% of 48% (by volume) acid and water and being characterized by freedom from any constituent which impairs the effectiveness of said acids in promoting the formation of such protective film.

10. The treatment of articles made from corrosion resistant ferrous alloys such as alloys of the l88 type in a bath consisting of 120% chromic acid, l-10% commercial hydrofluoric acid and water.

11. The treatment of articles made from corrosion resistant ferrous alloys such as high chromium ferrous alloys and high chromium-nickel ferrous alloys and after the same have been subjected to treatment for the removal of scale, which consists in subjecting such articles to the action of a hath made up of from 1-20% chromic acid, from l10% commercial hydrofluoric acid and water.

12. A method which consists in immersing an article made from a ferrous alloy containing about 18% chromium and about 8% nickel in a bath containing, besides water, only 1-20% chromic acid and l-10% commercial hydrofluoric acid for the purpose of providing a film on such articles which will prevent them from being sub ject to localized corrosive attack.

13. A treatment for the purpose of preventing localized corrosive attack of articles made from ferrous alloys having a relatively high chromium content or a relatively high chromium and a relatively high nickel content, which consists in subjecting such articles to the action of a bath made up of from 1-20% chromic acid, from 1-5% commercial hydrofluoric acid and water at a temperature between about 120 F. and 180 F.

14. A bath for immunizing against localized corrosive attack corrosion resistant ferrous alloys, said bath consisting of 1-20% chromic acid, 1-l0% commercial hydrofluoric acid and water and being free from any constituent which impairs the eifectiveness of said acids in providing such immunization.

15. A method of immunizing against localized corrosion or pitting, corrosion resistant chromium and chromium nickel steels having a discontinuous natural film and surface intrusions which comprises eliminating said surface intrusions and causing the film to become continuous and improved as to its characteristics by subjecting the same to a bath consisting of chromic and hydrofluoric acids in water.

16. A bath for rendering corrosion resistant ferrous alloys immune to localized corrosion or pitting which consists of about l-20% chromic acid and about .55% hydrofluoric acid, the balance being substantially all water.

CHARLES A. SCHARSCHU.

GEORGE C. KIEFER. 

